Head-up displays (HUDs) were introduced in automobiles by GM in 1988. In the past decade they have been offered by several automakers, although covering only a small niche of the market. They are now available in many high end car models from various manufacturers, as standard or optional features, thanks to significant R&D progress during the last decade. However, in contradiction to various forecasts, HUDs have still not penetrated the automotive market nearly as much as expected [1]. Automotive safety has been stated as the main driving force for implementing HUDs, which are capable of displaying information without requiring the user to take their eyes off the road, e.g. content from night vision cameras, navigation systems, and others. Studies of driving behavior found that improper lookout, inattention and distraction [2], as well as “eyes-off-road durations of greater than two seconds”, have significant impact on crash risks [3]. An analysis of rear-end collision avoidance systems concludes that nearly 40% of drivers “appeared to be distracted within five seconds before the crash-imminent alert.”[4] Altogether, it has been found that keeping the driver's attention on the road is one of the biggest contributors to avoid accidents. This of course is strongly coupled with the driver's vision being aimed ahead of the vehicle, through the car's windshield.
For future HUD applications, researchers have proposed different scenarios based on advanced environmental recognition technologies to support and alert the user. For example, systems could highlight traffic signs or obstacles on the road, alerting the driver of what to expect ahead in all road conditions. These extended “augmented reality” versions may utilize parts or the whole windshield of the car to cover whole outside scenario. In contrast to “augmented reality”, the stated safety increase based on the HUD use is primarily derived from the fact that all necessary information from the dashboard is displayed on the windshield and that it is highly simplified and not overloading the driver [5]. Bringing all strings together leads to the conclusion that an automotive head-up display is intended to project much of the regular information the driver would otherwise get from the dashboard, displayed in a minimalistic and common way to minimize driver distraction. This information may include vehicle speed, gear, radio settings, and navigation information.
A light engine creates the necessary light with a monochrome or multi-color content for the projection. Recently, there has been increased focus on MEMS-based laser-scanning light engines due to their compact size and saturated color content capability [6]. This light is projected onto a screen surface, also termed the exit pupil expander or EPE [6], which is typically a diffuser plate placed at an adequate distance from the projector to achieve needed image size. This screen, typically several centimeters on the diagonal, is actually viewed by the driver due to the alignment of folding mirrors and lenses which make it visible “through” the windshield or a separate combiner plate. The optical path between the diffuser plate and the combiner may include a fairly complex set of folding mirrors and aspheric mirrors or lenses to achieve both a larger virtual image size as well as a greater perceived distance between the driver and the virtual image. In existing HUDs, total distance of the virtual image to the driver may be between 1.5 m to 2 m.
This type of display offers a number of very desirable properties, for example the long focal distance to the image for driver's additional comfort and the fact that the information is mostly available only to the driver and not viewable by others. On the other hand it is less efficient (electrical power to actual viewable optical brightness) and bears severe challenges which we will briefly outline. The image on the diffuser plate has to pass several mirrors and/or lenses, each of which result in some brightness loss. Furthermore, the final reflection from the transparent windshield or combiner plate is low (typically less than 25% reflected), i.e. the image transmitted through the windshield to the area above the vehicle is brighter than the one viewed by the driver. But the real challenges in brightness and inefficiency happen prior to this optical train. RGB lasers have low efficiency, especially green lasers, which limits the maximum brightness of the system. Laser combining optics which form a single laser beam, and MEMS projectors can have a further 50% loss. The diffuser plate can have efficiency below 50%.
Regarding the forming of the image content itself, there is an additional inefficiency. A typical non-distracting HUD image is mostly empty space and a small percent of the available image area contains content. Due to the various beam retrace considerations in laser raster projection (active video time vs. complete period of one raster) and the small amount of content, lasers are actually utilized only a few percent of the time. Given the upper limitation on power in the RGB lasers (e.g. 50 mW for green,) the fact that the laser is turned on for roughly ˜5% of the time to form images ˜10-20 times lower brightness. Finally, it is found that if drivers wear polarized glasses, the display may become invisible because of its strong polarization.
One alternative approach to the “virtual image HUD” described above is a “windshield display”, displaying content directly onto the windshield itself so that it can be viewed by the driver and other passengers. In 2007 we published [7] results of a project for an automotive customer to create a prototype windshield display system which could cover as much windshield area as possible. This concept was enabled by mostly transparent fluorescent emissive “Superimaging” films [8] that were applied to the surface of the windshield or embedded in a windshield. The films contain nano particles which are substantially transparent in visible wavelengths due to their small size, however when illuminated by a 405 nm laser beam they emit incoherently and in all directions at longer wavelengths, e.g. in blue or red colors. The result is that the windshield itself has readable content presented on it while otherwise remaining transparent. In this approach all of the optical losses associated with the forming of the virtual image at a distance beyond the windshield are avoided, resulting in superior brightness. Furthermore, the resulting image on the treated windshield is viewable from almost every angle, which removes the restrictive “eye-box” challenge for the drivers.
Regardless of whether the virtual image HUD or windshield display methodology is used, our proposal is to improve the efficiencies and driver feeling by employing a single, efficient laser source, replacing the diffuser plate methodology with emissive films or remote phosphors, and displaying vector graphics content instead of rastering bitmap images. The key advantages are listed below:
No speckle noise: One of the major problems with laser-based displays is speckle noise [9]. This phenomenon based on interference of the coherent laser light is a significant drawback which trumps many advantages of the laser-based optical sources and therefore a wide variety of methodologies is employed to reduce this effect [9]. The methodologies add design complexity and moving optical parts and while only reducing the problem. In our proposed methodology where images are generated incoherently on emissive materials, speckle noise is virtually non-existent.
No polarization dependence: Unlike laser based picoprojectors or LCDs, the phosphor-emitted images are not polarized and eliminate the dimming issue for users wearing polarized (sun) glasses.
Higher efficiency and lower cost laser sources: Lasers at the 405 nm and 445-450 nm wavelengths are widely used in blue ray DVDs and other applications and have become widely popular in 3D laser printing systems and many other applications where efficient laser sources are needed at a highly competitive consumer price point. Typical efficiency of a single mode 405 nm laser diode with approx. 200 mW of output power is 20% (˜5V and 200 mA). As a comparison, green laser diodes used in picoprojector type HUDs achieve at most 5% and remain very costly.
Less complex optical design: A single (color) laser diode source requires very simple optics without any dichroic mirrors and combiners as used to combine red, green, and blue lasers into a single co-axial beam. RGB lasers are not only difficult to align (especially over automotive temperature range) while reducing optical efficiency, but they are also relatively costly. With a single laser, complex color control hardware and algorithms can also be avoided.
High optical resolution: The laser based display with 405 nm wavelength can have a higher optical resolution than an RGB based display, due to the shorter wavelength laser, especially if compared with RGB's red wavelength of ˜638 nm. In the case of vector graphics this results in increased image sharpness and clarity.